Method of applying phosphate conversion coating by reverse roller technique

ABSTRACT

An acidic aqueous coating solution particularly suited for application to ferriferous surfaces by a reverse roller coating process, that is a process wherein the coating solution is applied to a moving metal surface by means of a feed roller which is rotating in a direction opposite to that in which the metal surface is moving, the solution being effective to produce a phosphate conversion coating on the ferriferous surface and containing alkali metal phosphate or phosphoric acid and chlorate, the ingredients being present in relative amounts which are dependent on the pH of the solution which is within the range of about one to about 3.5.

United States Patent [72] Inventor Richard A. Montella Ambler, Pa.

[21] Appl. No. 714,217

[22] Filed Mar. 19, 1968 [45] Patented Oct. 26, I971 [73] Assignee Amchem Products, Inc.

Ambler, Pa.

[54] METHOD OF APPLYING PHOSPHATE CONVERSION COATING BY REVERSE ROLLER TECHNIQUE 10 Claims, 1 Drawing Fig.

[52] U.S. Cl l48/6.15 R,

[51] Int. Cl C2317/10 [50] Field of Search 148/6.15,

[56] References Cited UNITED STATES PATENTS 1,780,566 11/1930 Pedersen 117/49 2,293,716 8/1942 Darsey 148/6.15

Primary Examiner-Alfred L. Leavitt Assistant ExaminerJ. R. Batten, .lr. Attorney-Synnestvedt & Lechner ABSTRACT: An acidic aqueous coating solution particularly suited for application to ferriferous surfaces by a reverse roller coating process, that is a process wherein the coating solution is applied to a moving metal surface by means of a feed roller which is rotating in a direction opposite to that in which the metal surface is moving, the solution being effective to produce a phosphate conversion coating on the ferriferous surface and containing alkali metal phosphate or phosphoric acid and chlorate, the ingredients being present in relative amounts which are dependent on the pH of the solution which is within the range of about one to about 3 .5.

METHOD OF APPLYING PHOSPHATE CONVERSION COATING BY REVERSE ROLLER TECHNIQUE This invention relates to producing phosphate conversion coatings on metal surfaces. More particularly, this invention relates to phosphate-containing conversion coating solutions and to the application of such solutions to ferriferous metal surfaces by the reverse roller coating technique. The source of the phosphate ion in the coating solutions to which this invention relates is an alkali metal phosphate, including ammonium phosphate, or phosphoric acid.

It is known to apply to ferriferous metal surfaces acidic phosphate-containing coating solutions which form thereon coatings which are corrosion resistant and paint adherent. For quite a long time, such solutions have been applied and are applied presently to the metal surface by the conventional methods of immersing the surface in a bath of the coating solution or spraying the solution onto the surface.

A fairly recent innovation in the art relating generally to the application of conversion coating solutions to metal surfaces is the application process often referred to as the reverse roller coating technique. It is described and claimed in U.S. Pat. No. 3,098,775 and 3,215,564 to Thirsk, assigned to the same assignee as this application. In this coating application process, the coating solution is applied to a moving metal surface by means of a feed roller whose surface is wetted with the coating solution and is rotating in a direction reversed or opposite to that in which the metal surface is moving past the roller surface. This coating technique is particularly suited to applying coating solution to metal in strip form.

The reverse roller coating technique has a number of advantages over other techniques for applying conversion coating solutions to metal surfaces. For example, with respect to that method of applying coating solution by moving the metal surface past or over a roll or rollers which are rotating or moving in the same direction as the metal surface, the reverse roller coating technique has the advantages that the coatings can be applied to the surface with greater rapidity and without sacrificing coating quality; in many applications, an improvement in coating quality is obtained. These advantages are realized also when the reverse roller coating technique is compared with other application methods, such as spraying and immersion methods.

Although a variety of types of conversion coatings of good quality can be formed on a variety of types of metal surfaces by applying the conversion coating solution to the surface by the reverse roller coating technique, I have found that when utilizing this technique in forming a prior art phosphate conversion coating on ferriferous metal surfaces, the advantages over the conventional spray and dip systems tend to be attenuated by certain disadvantages. For example, it has been found that when conventional phosphate conversion coating solutions are applied to ferriferous surfaces by the reverse roller coating technique, the usefulness of the coatings obtained as paint bases does not always match that of coatings produced when the same solutions are applied by a conventional spray process.

Another problem that I have encountered in applying conventional phosphate-containing conversion coating solutions to ferriferous surfaces by the reverse roller technique is that the dwell time is relatively long. This adversely affects the economics of the coating process. The phrase dwell time refers to the time period during which the coating forms on the surface and is measured from the time the solution is applied to the moving metal surface by the coating roller to the time excess or spent solution is removed from the surface, as by rinsing or squeegeeing.

The interrelated factors utilized in building into the coating process a dwell time period are the speed of the metal surface and the position of the equipment for removing excess of unused solution relative to the position of the coating roller. Thus, if the dwell time is relatively long and the metal surface is moved at a relatively fast speed, the equipment for removing the excess solution must be positioned relatively far away from the coating roller, in which event plant space requirements are increased. On the other hand, if the equipment is positioned closer to the coating roller to decrease plant space requirements, the speed of the metal surface would have to be slowed down to provide for the necessary dwell time period, in which event production would be reduced. It can be seen that a coating solution that requires a relatively long dwell time can tend to offset the high speed application advantages offered by the reverse roller coating process.

In order to fully exploit the advantages offered by the reverse roller coating technique, the dwell time for an industrial application process operating with satisfactory ferriferous surface speeds should be no longer than about 30 seconds and most preferably should be shorter.

In view of the problems that I have encountered, as outlined above, this invention is directed to the provision of a phosphate-containing conversion coating solution having properties which reduce the severity of or avoid these problems and which allow the solution to be used in a reverse roller coating process which is operated at satisfactory industrial speeds with a dwell time of no longer than about 30 seconds.

Over the years there have been a number of developments relating to phosphate-containing conversion coating solutions. However, none of them have been directed to providing a solution particularly suited to being applied to ferriferous sur' faces by the reverse roller coating technique nor have they produced by solution which inherently has properties that are effective in economically forming a quality coating when applied to ferriferous surfaces by this technique. For the purpose of background, some of these developments, as described in U. S. Patents, are summarized below.

In U.S. Pat. No. 2,479,423 there is described a phosphatecontaining conversion coating solution which is said to have improved corrosion and cleaning properties, as well as coating properties. The aqueous coating solution contains dissolved alkali and ammonium phosphates and undissolved ferric or aluminum phosphates, and can also contain an emulsifying agent. The pH of the coating solution ranges from 5 to about 5 and it is emphasized that the pH should be kept within this range to obtain optimum performance.

In U.S. Pat. No. 2,502,441, there is disclosed a conversion coating solution containing an acid salt of phosphoric acid and, as additives, a phenolic compound and a water soluble molybdenum or tungsten compound. It is said that the additives accelerate the production of the conversion coating and also enhance the quality and consistency of the coating obtained. The pH of the coating solution is given as 3.2 to 5.5.

Another modification to phosphate conversion coating solutions is disclosed in U.S. Pat. No. 2,609,308. The solution consists of an acid phosphate of an alkali metal and an oxidizing agent, including nitrites, sulfites, chlorates and bromates of the alkali metals. The solution is said to produce coatings of increased thickness and improved paint adherence. It is disclosed also that the pH of the solution should not be substantially less than 4.2 if good paint adhesion properties are to be obtained and 6 is set as the upper pH limit.

In U.S. Pat. No. 2,665,231 there is disclosed a phosphate coating solution containing an alkali metal phosphate and an alkali metal fluoride. The coating solution is described as effective in minimizing the dissolution of iron, thereby reducing the tendency of sludge formation, and in eliminating corrosive action of the solution on equipment. It is further stated that the pH of the solutions should be between 3 and 5.8 because a lower pH gives too much pickling and a higher pH stops all pickling action.

There is disclosed in U.S. Pat. No. 2,724,668 a conversion coating solution containing an alkali metal phosphate and an alkali metal lignosulfonate. It is stated that the solution is capable of, not only forming a protective phosphate coating on the metal surface, but also of cleaning the surface when the solution is applied by spraying. Oxidizing agents, such as those disclosed in U.S. Pat. No. 2,609,308 above, may also be added to the solution. The pH of the solution is said to range from about 4.2 to about 6.

in US. Pat. No. 2,758,949 there is disclosed the addition of a sequestering agent, including polyphosphates and certain amino acids, to alkali phosphate coating solutions, which preferably contain an oxidizing agent, such as a chlorate, bromate, sulfite, etc. it is said that the sequestering agent deters the formation of spots and dust on the resultant coating. The pH of the solution is stated to be between 3 and 6.

In US. Pat. No. 2,954,309 there is disclosed a conversion coating solution containing alkali metal dihydrogen phosphate, a pyrophosphate, and a soluble metal compound of either arsenic or chromium. The pH of the solution is said to be in the range of 4.1 to 6.5. Advantages of such a solution are said to be the formation of minimal amounts of sludge and the capability of being applied equally well by spray or immersion methods.

From the above patents a few general observations can be made. The patents disclose a variety of coating solutions, which in addition to containing phosphate, also contain one or more additives for the purpose of obtaining an improved or specialized effect. Also, all of the coating solutions are acidic and the overall pH range is 3 to 6.5. (It is noted that in industrial practice, the solutions generally are operated at a pH between 4.2 and 5.8, the specific pl-i within this range being dependent on the chemical makeup of the solution utilized.) It is further noted that none of the patents is directed specially to a coating solution particularly suited to being applied to a ferriferous surface by the reverse roller coating technique.

Whereas the reverse roller coating technique can be utilized to apply the above described coating solutions to ferriferous surfaces to form thereon conversion coatings, none of them possesses properties which provide both for formation of the high quality coatings expected by industry and for the high speed production desired for economic reasons. The overall result of this is that in the art of forming phosphate conversion coatings on ferriferous surfaces, it has not been heretofore possible to realize in an industrial scale process all of the advantages that are obtained by being able to apply the coating solution to the surface by the reverse roller technique.

it is therefore an object of this invention to provide a phosphate-containing conversion coating solution particularly suited for being applied by the reverse roller coating technique to a ferriferous surface.

it is another object of this invention to provide an improved process for applying by the reverse roller coating technique a phosphate-containing conversion coating solution to ferriferous surfaces.

It is another object of this invention to provide a phosphatecontaining conversion coating solution having properties which enable the solution to be utilized in a reverse roller coating process for applying good quality coatings to a fer riferous surface wherein the ferriferous surface is moved at a relatively high speed and wherein the dwell time is no longer than about 30 seconds.

The above objects and others are attained in accordance with this invention by utilizing in a reverse roller coating process an acidic aqueous coating solution comprising as essential coating-producing ingredients phosphate and chlorate, wherein the pH of the solution can be substantially lower than heretofore utilized and wherein the relative amounts of phosphate and chlorate utilized in the solution are dependent on the pH of the solution and are interrelated therewith to form a phosphate conversion coating on a ferriferous surface.

The pH of the acidic coating solution of this invention can be as low as about 1 and is within the range of about 1 to about 3.5. When the pH of the solution is within the lower portion of the range, the amount of phosphate ion must be at least 5 g./l. and the amount of chlorate ion must be at least about 3 g./l. On the other hand, when the pH of the solution is within the higher portion of the range, the amounts of phosphate and chlorate must be at least about 20 g./l. and about 12.5 g./l. respectively. in general, but with some qualifications as will be explained more fully below, the minimum effective amounts of phosphate and chlorate that can be utilized increase as the pH of the solution increases.

As illustrative of the relative minimum amounts of phosphate and chlorate that can be used as the pH of the coating solution is varied, there is set forth in the table below exemplary solutions within the scope of this invention.

pose of showing how the pH of the coating solution influences the minimum amounts of coating-producing ingredients that can be utilized.

Prior to discussing in detail the pH-minimum amount relationship, it is first noted that the source of the phosphate ion in the solutions of this invention can be phosphoric acid or an alkali metal phosphate which when used herein and in the claims includes within its meaning ammonium phosphate, and also alkali metal and ammonium monohydrogen and dihydrogen phosphates. The source of the chlorate ion can be any water soluble chlorate-containing compound which has a cation compatible with the other ingredients of the solution and has no deleterious effects on the coating solution or on the coating formed. The chlorate is preferably added to the solution in the form of an alkali metal chlorate which when used herein and in the claims includes ammonium chlorate. Most preferably, the chlorate is added in the form of sodium, potassium or ammonium chlorate.

in accordance with this invention and with reference to the above table, it has been found that when the solution has a pH within the range of from about i to less than about 2.3, the amounts of phosphate and chlorate ions must be at least about 5 g./l. and about 3 g./l. respectively in order to produce acceptable coatings on ferriferous surfaces when the coating solution is applied to the surface by the reverse roller coating process. Examples 1, 2 and 3 of the table are illustrative. Thus excellent coatings can be produced quite economically by utilizing such amounts when the coating solution is within the range of about 1 to less than about 2.3. Higher amounts of phosphate and chlorate can be used, but as will be explained more fully below, if the amounts of the coating-producing in gredients exceed certain amounts, economic disadvantages are incurred.

Examples 4-7 of the above table are set forth to show that when the pH of the solution is within the range of about 2.3 to about 2.8, the minimum amounts of phosphate and chlorate that can be utilized must be at least about 5 g./l. and about 3 g./l. respectively, provided that when one of the ingredients is present in an amount in the range of its minimum amount, the amount of the other ingredient must be greater than its minimum amount, and to the extent that when the chlorate is present in its minimum amount, that is about 3 g./l., the amount of the phosphate must be at least about 20 gJl. On the other hand, if the minimum amount of phosphate is utilized, that is about 5 g./l., then the amount of chlorate that is utilized must be at least about 12.5 g./l.

Within the pH range of about 2.3 to about 2.8, there can be formulated effective coating solutions which contain amounts of phosphate and chlorate greater than about 5 g./l. and about 3 g./l. in other words, it is not necessary that one of the coating-producing ingredients be present in an amount in the range of its minimum amount. in formulating such solutions, it has been found that if more than about 5 g./l. of phosphate are utilized, the minimum amount of chlorate utilized can be lower than about l2.5 g./l., but in no event should it be below about 3 g./l., even if quite large amounts of phosphate are utilized. Similarly, if more than about 3 g./l. of chlorate are utilized, the minimum amount of phosphate utilized can be below about g./l., but in no event should it be below about 5 g./l., even if quite large amounts of chlorate are utilized. In formulating such solutions, that is solutions wherein both the phosphate and chlorate are present in amounts greater than about 5 and about 3 g./l. respectively, the relative minimum amounts of each of the ingredients can be detennined quite quickly by following the rule that as the concentration of one of the ingredients increases, the concentration of the other ingredient can be decreased. For example, as the concentration of phosphate is increased from 5 g./1. on up to say about 20 g./ l., the minimum effective concentration of chlorate can be decreased from about 12.5 g./l. on down to about 3 g./l. Similarly, if the concentration of chlorate is increased from about 3 g./l. on up to about 12.5 g./l., the minimum effective concentration of phosphate can be decreased from about 20 g./l. on down to about 5 g./l. Having utilized a predetermined amount of one of the ingredients, following the guidelines set forth above, adjustments in the amount of the other ingredient can be made rather easily if it is desired to operate the solution with the minimum concentration of the other ingredient.

In accordance with this invention, it has also been found that in applying by the reverse roller coating process a coating solution having a pH within the range of greater than about 2.8 to about 3.5, the minimum amounts of both the phosphate and chlorate needed for the formation of acceptable and quality coatings must be greater than about 5 g./l. and about 3.0 g./l. respectively. As the pH is increased from greater than about 2.8 to about 3.5, it is necessary that the minimum amounts of both phosphate and chlorate utilized be increased and to the extent that when the pH of the solution is about 3.5, the minimum concentration of the phosphate be about 20 g./l. and the minimum concentration of the chlorate be about 12.5 g./l. (See example 8 in the table above.) It is noted that these amounts are the minimum effective amounts of phosphate and chlorate and thus higher amounts can be used.

It should be appreciated that when operating with a coating solution having a pH within the range of greater than about 2.8 to about 3.5, the specific pH will determine the minimum effective amounts of phosphate or chlorate that can be used, such minimum amounts always being greater than 5 g./l. of phosphate and 3 g./l. of chlorate, but for pHs less than 3.5, such minimum amounts being below 20 g./l. of phosphate and 12.5 g./l. of chlorate.

Although the above-described relationship between the minimum amounts of chlorate and phosphate and the pH of the coating solution has been found to exist for solutions which are applied to ferriferous surfaces by the reverse roller coating technique, the relationship does not necessarily apply or hold true for phosphate-chlorate solutions which are applied by other methods, such as the spraying and immersing methods.

If amounts of phosphate and chlorate below the minimum amounts set forth above are used, the quality of the coating that is formed is rather poor.

In general, amounts of phosphate and chlorate utilized in the coating solution should not exceed about 20 g./l. and about 12.5 g./l. respectively. From the standpoint of forming a quality coating on the ferriferous surface, greater amounts are not necessarily detrimental and higher amounts can be utilized, such higher amounts being limited only by the solubility in the aqueous solution of the phosphate and chlorate compounds. However, when considering the economic aspects of the application process, it is important that the use of amounts of phosphate and chlorate in excess of about 20 g./l. and about 12.5 g./l. respectively be avoided. This is because the use of such higher amounts adds to the cost of the chemical constituents used in the coating solution, thereby adversely affecting the economics of the application process. Furthermore, in view of my finding that lower amounts of phosphate and chlorate are effective in providing a quality conversion coating, the use of higher amounts is not particularly justified.

It is noted that the reverse roller coating technique can be utilized to apply phosphate-chlorate-containing coating solutions to ferriferous surfaces to form thereon a phosphate conversion coating, even if the pH of the solution is greater than about 3.5. However, if quality coatings are to be produced most economically, it is mandatory that the pH of the coating solution be no higher than about 3.5. This is because to produce quality coatings at a pH of greater than about 3.5 the amounts of phosphate and chlorate that must be used in the coating solution are so high and the dwell time so long that the overall cost of the process tends to be high. These factors cause disadvantages which more than offset the advantages inherently present in the reverse roller coating application technique.

it is preferred that the coating process be operated with a solution having a pH in the lower ranges of about 1.0 to about 2.8. This is because lesser amounts of ingredients can be utilized in this range; moreover, it has been found that as the pH of the solution increases, the dwell time must be increased.

The use of coating solutions having a pH of less than about I are to be avoided because the coating obtained is not a good base for siccative finishes.

Most preferably the application process is operated utilizing a coating solution having a pH ranging between about L8 and about 2.8. This is because relatively small amounts of the chemical constituents comprising the solution can be utilized to produce coatings that are excellent bases for final finishes. Moreover, such coatings can be produced within a relatively short dwell time.

When it is desired to use an aqueous coating solution having a lower pH value, for example between about 1.0 and 2.3, the hydrogen ion content may be supplied by acids other than phosphoric. Thus, other mineral acids, such as for example sulfuric, nitric or hydrochloric, may be added to the coating solution in amounts sufficient to impart to the solution the pH desired.

The solutions described herein can be applied by the reverse roller coating technique to any ferriferous surface for the purpose of forming thereon good quality coatings. The term ferriferous" as used herein includes within its meaning surfaces such as iron, steel and alloys of iron.

With reference to the drawing, there is shown diagrammatically therein apparatus that can be utilized to apply the coating solutions of this invention to ferriferous surfaces according to the reverse roller coating technique. The details of construction of the rollers and the means by which they are moved and by which the strip is moved are not shown because they are well-known.

The ferriferous strip S is shown as being moved in the direction of the arrows by suitable means not shown. As areas of the strip come into contact with coating roller 2, which is being driven in the direction of the arrow 2A, coating solution is applied to the strip. The coating solution 3 is transferred from the container 4 to the surface of the coating roller 2 by roller 6, which is submerged in the coating solution, and the roller 10. Each of these rollers is moving in the direction of the arrows shown.

It it is desired to apply coating solution to the underside of the metal strip, this can be done by the coating roller 12 which is in contact with the underside of the strip and which is being driven in a direction opposite to that in which the strip is moving. Conversion coating solution 13 is transferred from its container 14 by the roller 15 to the coating roller 12 as shown.

After an appropriate dwell time, solution that remains on the strip must be removed therefrom. This is because if soluble salts are allowed to deposit and remain, the ultimate life of a paint finish applied to the resultant coating tends to be shortened. it will be appreciated that some of the solution will tend to drip from the strip into the collecting pan P. However, means should be provided to remove the remainder of the excess or spent solution. As shown in the drawing, squeegee rollers 29 and 30 are provided ro this purpose. Other appropriate means can be utilized to remove solution that remains on the strip, such as for example, a water rinse. it should be appreciated that if the nature of the coating solution is such that an inordinately long dwell time is necessary to allow for the formation of a quality coating, the economics of the process are adversely effected. This is because space requirements for the coating apparatus must be increased or the overall time for completing the coating process is increased. In accordance with this invention, the coating solutions described herein have properties such that the dwell time for any particular coating solution need not be greater than about 30 seconds, which is a tolerable time period in that it does not appreciably add to the cost of the application process. it should be understood that longer dwell time periods than 30 seconds can be used, but without particular benefit.

With respect to the minimum dwell time, this will vary from solution to solution. After the solution is applied, some period of time will elapse before the coating is formed. Therefore, some time period must be provided for before means are utilized to remove from the ferriferous surface the solution that has not taken part in forming the coating. In general, at least a 4-second dwell time period will be required. However, this may vary depending on the particular solution utilized and the minimum dwell time can be longer or shorter than 4 seconds. ln accordance with this invention it need not be longer than about 30 seconds.

Another aspect of this invention is the finding that improvements in the coating application process are obtained if the ferriferous surface is heated somewhat so that as the surface comes into contact with the coating roller surface, the temperature of the surface is above ambient, but is not so hot that it will cause the coating solution to dry before it reaches the means provided for removing excess coating solution. An advantage obtained by applying the coating solution to a worm ferriferous surface is a reduction in the dwell time required to give optimum coatings. The problem encountered if the surface is so warm that the coating dries prior to being exposed to the means for removing excess or spent coating solution is that this solution will be more difficult to remove. it has been found that if the ferriferous surface has a temperature ranging between about 130 F. and about 140 F. at the time the coating solution is applied by the coating roller, particularly good results are obtained. The heating of the ferriferous surface may be accomplished by any suitable means, for example by rinsing the surface with water hot enough to relatively quickly raise the temperature of the surface to somewhere between about 130 F. and about 140 F.

In the reverse roller coating technique, the amount of the solution that is applied to the strip is related to the speed of the surface of the coating roller and the opposing linear speed of the metal surface, other factors held constant. Thus, as the ratio of the roller surface speed to the linear speed of the metal surface increases more coating solution is made available to the area of the metal surface when it contacts the roller surface. This ratio of the roller surface speed to the opposing linear speed of the metal surface is generally expressed as a percentage. For example, if the roller surface speed is 120 ft./min. and the linear speed of the metal surface is 90 ftJmin. the ratio of the roller surface speed to the metal surface speed would be 120/90 or about 133 percent.

Another aspect of this invention is that the coating solutions described herein are capable of forming acceptable coatings on ferriferous metal surfaces at roller surface/metal surface speed ratios of from as low as about 25 percent to as high as about 175 percent. This is an important characteristic of the coating solutions described herein because of the flexibility with which the rate of feed of the coating solution can be controlled. This characteristic is utilized to advantage when coat ing different types of ferriferous surfaces or strips, for example strips that are good" or easy" to coat, that is strips which readily accept a coating or strips which are bad or difficult to coat, that is strips which do not readily accept a coating. With respect to "good strips, roller surface/metal surface linear speed ratios as low as 25 percent can be utilized, whereas with bad" strips, the ratios utilized can be upwards of 100 percent.

Set forth hereinafter are specific examples illustrating the use of this invention. In all of these examples, cold rolled mild steel strip was treated in the following general sequence:

1. alkaline cleaning at 160 F.

2. cold water rinse 3. cold water rinse 4. water rinse at 130 F.-140 F.

5. application of phosphate-chlorate aqueous solution by reverse coating roller 6. cold water rinse for removing excess solution 7. acidulated chromium-containing final rinse at 140 F.

The alkaline cleaner used in stage (1) was applied by spray means. Stages (2) and (3) utilized old water spray means while stage (4) applied a warm water rinse by spray means. The water in this stage was kept just warm enough so that when coating solution was applied at stage (5), the heat did not cause the treated steel to dry prior to reaching the rinsing stage (6). In other words, the water in rinse stage (4) was not so hot as to cause drying of the steel strip during the dwell time required for formation of the phosphate coating. in stage (7) there was utilized an acidulated rinse containing partially reduced hexavalent chromium at about a 0.07 percent concentration on a volume to volume basis.

After the treated strip was dried and prior to cooling, representative sections were removed and painted with one coat of acrylic paint (Duracron 100, manufactured by Pittsburgh Plate Glass Co., Pittsburgh, Pa.) and cured at 350 F. for 30 minutes.

To test the corrosion resistant properties of the coated strip salt spray tests were carried out in accordance with ASTM-B-l 17-64.

The impact tests for determining adhesion were carried out by dropping a i-inch ball with a force of -inch pounds on the painted test panel, usually on the reverse side, and then checking for peeled and/or cracked paint.

EXAMPLE I An aqueous coating solution was prepared so as to contain:

POE-added as phosphoric 5 grams acid (75%) Cl0,added as sodium 3.1 grams chlorate Water, to make 1.0 liter This solution, having a pH of 1.8, was used in a reverse roller coating machine to coat steel strip moving at 26 ft./min. The ratio of the reverse roller surface speed to' the steel strip speed was percent and the dwell time was 10 seconds. After the strip was treated in stages (1) to (7) sections were removed, cut into panels, painted an then subjected to 336 hours of salt spray testing as described above. Following this test period, the panels were examined and essentially no corrosion failure was observed. Other test panels were impact tested for adhesion with no failure being observed.

EXAMPLE ll Another aqueous coating solution was prepared containing:

PO} added as phosphoric 9.3 grams acid (75% ClO,'-added as sodium 3.1 grams chlorate Water, to make 1.0 liter The pH of the solution was adjusted to 2.2 with soda ash and used in the reverse roller coating machine to treat steel strip in substantially the same manner as in example I. Here again the strip moved at a speed of 26 ftJrnin. and the ratio of the reverse roller surface speed to the strip's speed was 125 percent. The dwell time was 10 seconds. The coated and painted strip was exposed to salt spray tests for 336 hours. No corrosion failure was observed, nor was there any adhesion failure evidenced after impact testing.

EXAMPLE III Another aqueous coating solution was prepared containing:

PO added u disodium 20 grams phosphate ClO,'added as sodium 12.5 grams chlorate Water, to make 1.0 liter The pH of the solution was adjusted to 3.5 with sulfuric acid and used in the reverse roller coating machine in essentially the same manner as those of the previous examples. However, a reverse roller to line speed ratio of 175 percent was utilized and the dwell time was 15 seconds.

The above examples have been set forth to illustrate specific coating solutions and the use thereof as encompassed by this invention and they should not be construed as limiting the invention. As heretofore mentioned, the coating solution of this invention can contain ingredients other than the chlorate ion and the phosphate ion coating-producing ingredientsacids other than phosphoric being examples of such other ingredients. In addition, other constituents can be added to the coating solutions for the purpose of obtaining specialized effects, surfactants and wetting agents being examples.

lclaim:

1. ln the process of forming a phosphate conversion coating on a ferriferous surface wherein a phosphate-containing acidic aqueous conversion coating solution is applied to the ferriferous surface by moving it past a roller surface moving in a direction opposite to the direction of the ferriferous surface, the improvement of utilizing an acidic aqueous conversion coating solution consisting essentially of at least about g./. of phosphate, the source of phosphate being a phosphate-containing compound selected from the class consisting of phosphoric acid and alkali metal phosphate, and at least about 3 g./l. of chlorate, the pH of the solution being within the range of from about 1 to about 3.5, the amounts of said ingredients in the solution and the pH of the solution being interrelated to form said conversion coating on said surface, al-

lowing the solution to remain on the ferriferous surface for at least a period of time until said coating is formed, but no longer than about 30 seconds, removing the solution remaining on the surface and thereafter applying paint to said conversion coating.

2. The process according to claim 1 wherein the pH of the solution is within the range of from about I to less than about 2.3.

3. The process according to claim 2 wherein the amount of phosphate ranges from about 5 g./l. to about 20 g./l. and wherein the amount of chlorate ranges from about 3 g./l. to about 12.5 g./l.

4. The process according to claim 1 wherein the pH of the solution is within the range of from about 2.3 to about 2.8 and wherein the amount of at least one of said ingredients is greater than the minimum amount set forth.

5. The process according to claim 2 wherein the pH of the solution is within the range of from greater than about 2.8 to about 3.5 and wherein both of said ingredients are present in amounts greater than their minimum amounts set forth.

6. The process according to claim I wherein the amounts of phosphate and chlorate do not exceed about 20 g./l. and about 12.5 g./l. respectively.

7. The process according to claim 1 wherein the solution is allowed to remain on the ferriferous surface for at least about 4 seconds.

8. The process according to claim 1 wherein at the time the coating solution is applied to the ferriferous surface, the ferriferous surface has a temperature above ambient but below a temperature which will cause the coating solution to dry before said means are utilized to remove solution from the surface.

9. The process according to claim 8 wherein the temperature of the ferriferous surface is in the range of about F.

to about F.

10. The process according to claim 1 wherein the pH of the solution is within the range of from about L8 to about 2.8.

s a s w t UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 5 5 3g 1 Dated October 2L 1971 flfljichard A. Montella It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE SPECIFICATION Column 1, line 70, "of"; second occurrence, should read -or--.

Column 2, line 38, "5'", second occurrence, should read --6-.

Column 3, line 26, "specially" should read --specifically--.

Column 6, line 73, "ro" should read --for--.

IN THE CLAIMS Claim 5, line 1, "2" should read --l--.

Signed and seals d this 25th day of April 1 972.

(SEAL) Attcst EDWARD M.FLETCHER, JR ROBETRT GOTTSCHALK Attesting Officer Commissioner of Patents :RM F'O-IOSO (10-69) USCOMM-DC 50376.pg9

HTS GOVERNMENT PRINTING OFFICE "6D 0-386-334 

2. The process according to claim 1 wherein the pH of the solution is within the range of from about 1 to less than about 2.3.
 3. The process according to claim 2 wherein the amount of phosphate ranges from about 5 g./l. to about 20 g./l. and wherein the amount of chlorate ranges from about 3 g./l. to about 12.5 g./l.
 4. The process according to claim 1 wherein the pH of the solution is within the range of from about 2.3 to about 2.8 and wherein the amount of at least one of said ingredients is greater than the minimum amount set forth.
 5. The process according to claim 1 wherein the pH of the solution is within the range of from greater than about 2.8 to about 3.5 and wherein both of said ingredients are present in amounts greater than their minimum amounts set forth.
 6. The process according tO claim 1 wherein the amounts of phosphate and chlorate do not exceed about 20 g./l. and about 12.5 g./l. respectively.
 7. The process according to claim 1 wherein the solution is allowed to remain on the ferriferous surface for at least about 4 seconds.
 8. The process according to claim 1 wherein at the time the coating solution is applied to the ferriferous surface, the ferriferous surface has a temperature above ambient but below a temperature which will cause the coating solution to dry before said means are utilized to remove solution from the surface.
 9. The process according to claim 8 wherein the temperature of the ferriferous surface is in the range of about 130* F. to about 140* F.
 10. The process according to claim 1 wherein the pH of the solution is within the range of from about 1.8 to about 2.8. 